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Equipment Discussions >> Mounts

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nexguy
member


Reged: 08/05/09

Loc: Wichita Falls, TX
What is this second mounting screw for on a CG-5? new
      #6180151 - 11/06/13 11:00 PM

The dovetail slot has two screws. What is the smaller silver one for?


Edited by nexguy (11/06/13 11:01 PM)


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orlyandico
Postmaster
*****

Reged: 08/10/09

Loc: Singapore
Re: What is this second mounting screw for on a CG-5? new [Re: nexguy]
      #6180156 - 11/06/13 11:07 PM

safety screw. Provide additional securing of your dovetail.

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core
Pooh-Bah
*****

Reged: 02/23/08

Loc: Mostly in Norman, OK
Re: What is this second mounting screw for on a CG-5? new [Re: orlyandico]
      #6180177 - 11/06/13 11:21 PM

... and there's actually a procedure to tighten them - when mounting scope, large knob first, then the silver one; you don't actually have to turn it down tight and put a blister on your fingers (the silver knob). Removing scope, loosen the silver one first, then the large knob. You'll notice that if you loosen the large knob first when removing scope, it will actually get harder to loosen the silver knob.

There's usually just a tiny bit of 'compression'/movement of the dovetail bar into the saddle when you first tighten the large knob. Tightening the silver knob slightly thereafter sets that silver knob in place. If you were loosen the large knob first, the dovetail/saddle 'springs' back/moves into the pre-stressed silver knob, and it'll dig just a bit more, making it harder to turn. Try it out for yourself.

See, we can even talk about proper techniques in mounting screws on a cloudynight


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: What is this second mounting screw for on a CG-5? new [Re: core]
      #6180191 - 11/06/13 11:25 PM

I didn't know about that procedure...

That said I think the practice of using one (or two..) screws to mount a dovetail is Bad Practice. We should all be using clamps.

Even the stock saddle on the CGEM uses a clamp..


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: What is this second mounting screw for on a CG-5? new [Re: nexguy]
      #6180198 - 11/06/13 11:31 PM

That's a safety set screw designed for the type of dovetail almost no one uses anymore. One pioneered by Vixen, copied by Synta, that has detents carved in the edges, and has largely been replaced with straight-sided Vixen style dovetails.

Example:

http://www.myastroshop.com.au/products/images/vixen-2661.jpg

Regards,

Jim


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jimbo728
sage


Reged: 11/05/13

Loc: Central NH
Re: What is this second mounting screw for on a CG-5? new [Re: jrbarnett]
      #6182150 - 11/08/13 01:17 AM

I still use that old Vixen plate because it holds the rings nicely on my old NightHawk and fits. The little set screw fits into a cut out. I tighten them both when using a regular Vixen rail for no other reason than they are there. Just a small measure of safety, I guess.

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Eddgie
Postmaster
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Reged: 02/01/06

Re: What is this second mounting screw for on a CG-5? new [Re: jrbarnett]
      #6182440 - 11/08/13 08:33 AM

Quote:

That's a safety set screw designed for the type of dovetail almost no one uses anymore.




Yes. Exactly.


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jrbarnett
Eyepiece Hooligan
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Reged: 02/28/06

Loc: Petaluma, CA
Re: What is this second mounting screw for on a CG-5? new [Re: jimbo728]
      #6182741 - 11/08/13 11:19 AM

I think it's a nice, secure dovetail design. Nothing wrong with it at all. The reason for its replacement with straight sided iterations is more likely cheapness. Casting or machining the more complex shape with the detents likely costs more than doing a simple straight-edged version. The straight edged versions, similarly, may be more versatile to accommodate a wider range of rings and other OTA fitments.

For a refractor of appropriate length for the dovetail's apcing, using rings, there's probably not a better V-series dovetail solution.

- Jim


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jrcrillyAdministrator
Refractor wienie no more
*****

Reged: 04/30/03

Loc: NE Ohio
Re: What is this second mounting screw for on a CG-5? [Re: jrbarnett]
      #6182756 - 11/08/13 11:27 AM

Quote:

I think it's a nice, secure dovetail design. Nothing wrong with it at all.




It is fine for use with tube rings, as the tube can be shifted within the rings to achieve balance. The smooth profile plates are required when the plate is directly attached to an OTA (commonly done with an SCT) because in that configuration the plate itself must move within the saddle to achieve balance.


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